(a) Field of the invention
The present invention relates to a telecentric zoom lens system for a measuring projector of which the diameter of the front lens is small, distortion is extremely small over a wide range of variation of magnification, and aperture ratio is large.
(b) Description of the prior art
A measuring projector is a measuring instrument of which the main purpose is to measure various values by illuminating the profile of an object to be measured by a suitable means, projecting the profile onto a screen enlarging it by means of a projection optical system of which aberrations, especially distortion, are corrected favourably, and making comparison with a reference figure on the screen. For the optical system to be used for the above-mentioned purpose, extremely high level of correction of distortion is required due to the character thereof and, moreover, such optical system should be arranged as a so called telecentric optical system, with which the principal ray on the object side of the rays which pass through the optical system becomes parallel with the optical axis in order to ensure the optical magnification accuracy at the time of focusing on the screen surface by moving the stage upward and downward. Besides, the optical system should have a high aperture efficiency because it is desirable that the luminance of the image is uniform over the whole area on the screen from the central portion to the marginal portion.
As the requirements for the optical system are strict as described in the above, these requirements are fulfilled at present by using a plural number of lens systems, which are individually designed to have fixed focal lengths suitable for respective magnifications, according to the purpose of application. Therefore, to change the magnification halfway during the measurement, it is indispensable to prepare a plural number of lenses with individually different magnifications, and it is essential to mount and dismount those lenses to the measuring projector as required or to exchange the lenses by a turret when the measuring projector is provided with the turret.
If it is possible to replace the above-mentioned plural number of lenses for a measuring projector with one zoom lens having sufficient vari-focal ratio, troublesome steps when changing over the magnification can be reduced, and, moreover, it is possible to make the measuring device as a whole compact and to reduce the total cost.
Besides, a zoom lens system enables to continuously vary the magnification by keeping the focusing plane fixed and, therefore, it is possible to observe and measure the shape of the object at any magnification in the range of variation of magnification of the zoom lens system.
However, almost no zoom lens system to be used for the above-mentioned purpose has been put to practical use. This is due to the reason that the tolerance in distortion required for this kind of optical system is small as explained before, and a technique for limiting distortion to a value within the tolerance has not yet been established for zoom lens systems, for which it has been considered to be especially difficult to correct distortion. Another reason is that the conventional method for giving the telecentric character to zoom lens systems has such effect that results in increase in the diameter of the front lens and, consequently, a compact optical system cannot be obtained and unfavourable influence is caused also to the aperture efficiency.
That is, known telecentric zoom lens systems are arranged that, in order to fulfill the condition for attaining the telecentric character, the zoom lens system comprises a varifocal lens system and master lens system, and an iris is arranged at the position of front focal point of the master lens system, which is a fixed lens group. Therefore, such zoom lens system does not comprise a lens group which is located in the rear of the iris and moved along the optical axis according to variation of magnification. For the above-mentioned type of zoom lens system, the diameter of the front lens should be made larger especially in the long focal length position when the moving range of movable lens group located in front of the iris is made larger in order to obtain high vari-focal ratio and, when it is attempted to make the diameter of the front lens small by keeping the vari-focal ratio unchanged, it becomes necessarily impossible to ensure a sufficient field angle.
There is a known zoom lens system comprising, for example, four lens groups respectively with positive, negative, positive and positive powers and arranged that the first and fourth lens groups are fixed while the second and third lens groups move along the optical axis in monotone patterns according to variation of magnification from the long focal length position to the short focal length position, the second lens group moving toward the first lens group at that time and the third lens group moving toward the direction opposite to the movement of the second lens group (for example, the lens system disclosed in Japanese published examined patent application No. 12424/76). With this type of zoom lens system, magnifications of the movable lens groups vary respectively in monotone patterns and variations of magnifications of respective movable lens groups effectively contribute to variation of magnification of the lens system as a whole, and it is therefore possible to make the movement amounts of respective lens groups small, and variations of aberrations during variation of magnification are small. Therefore, this type of zoom lens system is widely known as a type advantageous for arranging a high vari-focal ratio zoom lens system.
When arranging the above-mentioned type of zoom lens system as a telecentric optical system, an iris is generally arranged at the position of front focal point of the fourth lens group, which is a fixed lens group, as shown in FIG. 1. However, this arrangement causes a disadvantage that the entrance pupil comes toward the rear side (image side) when the zoom lens system is in the long focal length position. Generally, one of characteristics of projection optical systems is that the field angle does not become largely different even when the optical system has either a long focal length or short focal length. The diameter of the front lens thereof is determined chiefly by the position of the entrance pupil of the optical system. Therefore, in case of a zoom lens system arranged as described in the above, the diameter of the front lens becomes large when the entrance pupil in the long focal length position comes toward the rear side going away from the front lens. In other words, it is more advantageous when the iris is arranged at a position as close as possible to the front side. However, as far as the iris is located between the third and fourth lens groups, the position of the iris is limited to the range where the iris does not interfere with the third lens group when the zoom lens system is in the short focal length position where the third lens group comes closest to the fourth lens group. Besides, it is also advantageous for making the diameter of the front lens small when the focal length of the fourth lens group, which is the master lens group, is made long so that the inclination of the principal ray passing the center of the iris becomes small. In that case, however, such disadvantages are caused that the overall length of the lens system becomes long or the working distance becomes short. As explained so far, with the known method to fix the telecentric iris at the position of the front focal point of the master lens system, it is impossible to obtain an optical system which fully satisfies the requirements.